Electroluminescent (EL) devices hold the promise of providing a display technology superior to the cathode ray tube and liquid crystal displays in widespread use today. Although various EL devices have been known for years, development of EL displays has been relatively slow due to a number of technical challenges.
At least two different types of EL devices are known: tunneling EL devices and diode junction EL devices. Tunneling EL devices may be fabricated by placing a phosphor material between two electrodes and placing an insulating layer between one or both electrodes and the phosphor. Injection of carriers is accomplished by imposing a high voltage across the phosphor that enables tunneling of carriers, typically electrons, through the insulating layer. The high electric field accelerates the injected carriers that then interact with luminance centers within the phosphor resulting in emission of visible light.
Diode junction EL devices, on the other hand, are fabricated by doping the phosphor material to create a PN junction. Under forward bias conditions, holes and electrons recombine near the PN junction to emit light.
Both types of EL devices suffer from a number of problems, however, due to the high voltages required to inject carriers. High voltages in the phosphor region accelerate carriers to high velocities, such that many of the charge carriers pass quickly through the phosphor region without recombining or interacting with the luminance centers of the phosphor. The charge carriers do not contribute to electroluminescence, and hence, they create a wasteful leakage current, which lowers efficiency. Furthermore, high voltages can lead to catastrophic breakdown of the insulating layers or phosphor, destroying the device.
Fabrication of diode junction EL devices is also difficult since doping of most inorganic phosphors is difficult to achieve, limiting the choice of inorganic phosphor materials suitable for commercial EL devices. For those phosphors that can be doped, doping is typically limited to one carrier type, thus, limiting the efficiency of the devices.
Although the lower carrier mobility of organic phosphors can result in improved functionality, organic phosphors present a whole new set of difficulties. Organic phosphors tend to be highly chemically reactive and can rapidly degrade if exposed to the environment. The high reactivity of organic phosphors also limits the choices of materials that can be used for electrodes, since many organic phosphors will readily combine with the metal in the electrode, resulting in degradation of the device performance. Practical devices using an organic phosphor require special chemical isolation layers at the junctions and careful packaging to manage the reactivity of the phosphor. Achieving long life with organic phosphors has also proven difficult.